1. Field of the Invention
This invention relates to magnetic coatings, and more particularly to colorless, transparent magnetic coatings.
2. Description of the Prior Art
Those presently available magnetic coatings are based upon ferric oxide or chromic oxide. In many applications, such as magnetic tape, it is not a significant disadvantage that these materials are colored, i.e. ferric oxide having a brown color and chromic oxide displaying a green color. Nevertheless, there are many substantial applications for magnetic coatings which have been envisioned but wherein color is a significant disadvantage.
One such application is in the field of data bearing cards such as credit cards. These cards usually contain printed information and often contain a photograph of the card holder. The information contained on the face of these cards is covered by a clear, transparent plastic film such as polyvinyl chloride.
The desirability of coating such cards with magnetic stripes which could be encoded with information for machine readable credit cards has long been recognized. See, for example, Report and Recommendations of the Bank Card Standardization Task Force of the American Bankers Association's Bank Card Committee on Encoding Technology for Machine Readable Credit Cards, January, 1971. This report states many reasons why magnetic striping offers the greatest opportunity to reduce fraudulent credit card usage or satisfying other existing requirements. For example, magnetic data storage is proven technology which is reliable for indefinite periods of time and to some extent is existing technology. Magnetic striping also insures adequate data capacity for present and future needs. While no encoding technology would be foolproof against fraud, magnetic striping does offer a very high degree of security because it is difficult to alter, the data is not visible, and it requires a fairly high level of sophistication and collusion to counterfeit. Other advantages are that it can be applied at high speed, has great flexibility, is industrially compatible, is cost competitive, and is very durable.
Most suggestions to date have been to place the magnetic striping on the back of a credit card. This in large measure is due to attempts to avoid seriously defacing the logo, printed and/or photographic information on the face of most credit cards. Defacing results, of course, because presently existing magnetic coatings are non-transparent and colored.
Extensive research has, therefore, been undertaken to develop colorless magnetic materials. This research is evidenced by a large number of recently issued patents in the field. For example, Schafer, U.S. Pat. No. 3,399,957, discloses divalent europium magnetic materials which are transparent and crystalline; nevertheless, these magnetic materials are red. Wickham, U.S. Pat. No. 3,479,132, describes transparent magnetic compounds having a spinel crystal structure with a formula Li.sub.11 Fe.sub.6 V.sub.7 O.sub.32 ; these magnetic compounds are orange colored.
One patent, Vickery et al., U.S. Pat. No. 3,320,170, describes water-white transparent magnetic glasses formed from phosphate, borate or fluoride matrices containing rare earths such as gadolinium. These materials have not been generally commercially accepted, however. It is felt that the reasons for this are that the Vickery et al. materials have very low Curie points well within the range of ambient temperatures likely to be encountered with a credit card in normal handling and usage. Additionally, it has not been demonstrated that these materials have indices of refraction which match those of commercially available plastic matrices; this, of course, being required to produce transparent colorless plastic coating compositions. Lastly, it is felt that the Vickery et al. materials demonstrate only feeble magnetic properties.
It can be appreciated therefore that there is a great need for transparent, colorless magnetic solids which can be ground and dispersed in transparent plastic matrices to form magnetizable, transparent, colorless plastic coating compositions.